Method for quantitative determination of anions

ABSTRACT

A METHOD FOR THE DETERMINATION OF ANIONS IN SOLUTION BY MEANS OF ION EXCHANGE COMPRISING ADDING TO A SOLUTION, PREFERABLY AN AQUEOUS SOLUTION CONTAINING AN ANION IN UNKNOWN QUANTITY TO BE ANALYZED, A COMPOUND SELECTED FROM THE GROUP CONSISTING OF QUATERNARY ALIPHATIC AMMONIUM PICRATES HAVING A TOTAL OF AT LEAST 30 CARBON ATOMS, PREFERABLY DISSOLVED IN AN IMMICIBLE ORGANIC SOLVENT, THEREBY TO FORM BY ION EXCHANGE THE CORRESPONDING QUATERNARY AMMONIUM COMPOUND OF SAID ANION TO BE ANALYZED AND TO RELEASE A CORRESPONDING AMOUNT OF PICRATE ANION. SUITABLY TWO LAYERS ARE FORMED UPON MIXING AND THE QUATERNARY AMMONIUM COMPOUND OF SAID UNKNOWN ANION BECOMES DISSOLVED IN THE ORGANIC SOLVENT, WHILE SAID RELEASED PICRATE ION BECOMES DISSOLVED IN THE AQUEOUS PHASE. KNOWN TECHNIQUES SUCH AS OPTICAL METHODS ARE EMPLOYED TO DETERMINE THE AMOUNT OF RELEASED PICRATE ION, WHICH IS A MEASURE OF THE AMOUNT OF ORIGINALLL UNKNOWN ANION TO BE ANALYZED.

United States Patent 3,725,006 METHOD FOR QUANTITATIVE DETERMINATION OF ANIONS Arne Elof Brandstrom, Goteborg, Klas Ragnar Gustavii, Molndal, Ulf Krister Junggren, Pixbo, and Kerstin Kylberg, Molndal, Sweden, assignors to Aktiebolaget Hassle, Goteborg, Sweden No Drawing. Filed June 3, 1971, Ser. No. 149,766 Claims priority, application Sweden, June 4, 1970,

7,790/ 70 Int. Cl. B01d 11/04; C02b 1/60; G01n 31/04 US. Cl. 23-230 R 17 Claims ABSTRACT OF THE DISCLOSURE A method for the determination of anions in solution by means of ion exchange comprising adding to a solution, preferably an aqueous solution containing an anion in unknown quantity to be analyzed, a compound selected from the group consisting of quaternary aliphatic ammonium picrates having a total of at least 30 carbon atoms, preferably dissolved in an immiscible organic solvent, thereby to form by ion exchange the corresponding quaternary ammonium compound of said anion to be analyzed and to release a corresponding amount of picrate anion. Suitably two layers are formed upon mixing and the quaternary ammonium compound of said unknown anion becomes dissolved in the organic solvent, while said released picrate ion becomes dissolved in the aqueous phase. Known techniques such as optical methods are employed to determine the amount of released picrate ion, which is a measure of the amount of original unknown anion to be analyzed.

The present invention relates to a method for quantitative determination of anions by means of liquid ion exchange.

The object of the present invention is to determine anions quantitatively with precision.

During research done in order to obtain a rapid and accurate method for the determination of the fecal softener, sodium dioctylsulphosuccinate (Aerosol OT) in pharmaceutical preparations the general problem of determining anions was observed.

In the literature there are described a number of methods for such anion determinations based on different principles. The titration of anionic surfactants in a two-phase system has recently been described by Reid et al., Tenside 4, 292 (1967), and 5, 90 (1968). Sulfuric acids and inorganic anions have been extracted as ion pairs with different cations such as rodamine Zh, crystal violet, azure A, methylene blue and methyl green. A liquid anion exchange method [for the determination of perchlorate has also been disclosed.

A disadvantage of the methods mentioned is their empirical nature, which makes them unusable for practical applications or calculations of experimental conditions.

Recently a method for the extraction of barbituric acids and sulphonic acids by means of the quaternary ammonium ion methyl protriptyline has been described. The ion pairs formed are then determined photometrically or 'fluorometrically in the organic phase. In this case, the

precise reaction conditions have been calculated with equilibrium constants. The preparation of the reagent, however, is somewhat cumbersome, especially in large scale operations and because very stringent requirements on the purity of reagent and buffer substances have to be met to avoid coextraction of foreign anions.

Thus it has not surprisingly been found possible to avoid these disadvantages by means of the present invention, which comprises using at least one compound 3,725,006 Patented Apr. 3, 1973 selected from the group consisting of quaternary aliphatic ammonium picrates having a total of at least 30 carbon atoms as anion exchange compound. Preferably the total number of carbon atoms is 38, and preferably the aliphatic groups are alkyl, suitably alkyl chains having 6 to 8 carbons, and most preferably identical alkyl chains.

The n-tetrahexylammonium picrate has been found to be especially suitable but also n-tetraheptyl and n-tetraoctylammonium picrate can be used.

When an anion, which is to be determined photometrically after a liquid ion exchange, is added to a reagent of the formula Rom-06ml N6. the following reaction takes place: aq+ (R1 4 N C6 H2N3 dor-F 6 2 3 7 aq+ 4 org wherein X' denotes the anion in an aqueous phase which is to be determined.

It the quaternary ammonium picrate is present in excess and the reaction goes to completion the amount of picrate ions in the aqueous phase is a measure of the initial concentration of X.

Extraction Constant is defined as follows: Symbols:

X- is an ion P- is a picrate ion Q+ is a quaternary ammonium ion is concentration E is extraction constant aq is aqueous phase org is organic phase Addition of the reactions 1 and 3 give aq+Q or aq. +Q org which corresponds to the equation given above.

By means of a derivation via the extraction constants for the ion pairs included and the initial concentrations one will find that the concentration of picrate ions is equal to the sum of the concentration of free quaternary ammonium ions and the concentration of ion pairs between the quaternary ammonium ion and X. en this ion reaction is used for analytical purposes the analytical error is zero when the concentration of picrate ions is equal to the initial concentration of the anion X; which is equal to the sum of the concentration 0f the free anion X" and the concentration of (R NX in the organic phase.

A basis for the determination of the experimental conditions is optical properties, such as the light absorption or fluorimetric characteristics of the anion (C6H2N307) The molar absorptivity of the picrate ion in an aqueous solution is 1.4 10 at the absorbance maximum, 355 III/L. The most suitable concentration range for a sample is easily calculated to about 10 to 10* moles per litre when a 1 cm. photometric cell is used. The choice of reagent concentration is determined by the magnitude of the extraction constant for (R NX.

The quaternary ammonium picrate is preferably used in at least about 50 times excess, suitably at least about 100 times excess in comparison with the amount of anion which is to be determined.

The present invention will in the following be described with reference to the examples below, which, however, are not intended to restrict the invention.

EXAMPLE 1 Tetrahexylammonium picrate was prepared by shaking 1.5 moles (721.5 g.) of tetrahexylammonium iodide in methylene chloride with a first aqueous solution of 1.65 moles (343.5 g.) of picric acid adjusted to pH 8 with NaOH. When the phases had separated the aqueous phase was removed and a new aqueous solution containing 0.175 mole (40 g.) of picric acid adjusted to pH 8 with NaOH was added to the methylene chloride solution. The phases were shaken and allowed to separate. The aqueous phase was removed and the methylene chloride phase was washed with distilled water. Then the methylene chloride phase was evaporated and the residue, 800 g. of tetrahexylammonium picrate, was recrystallized from acetonitrile. Yield 92%. MP. 25-35 C.

EXAMPLE 2 The determination of the anion dioctylsulphosuccinate was made by shaking first 10.0 ml. of sodium dioctylsulphosuccinate dissolved in a phosphate buffer having pH 6.5 and the concentration of 10" moles per litre with regard of Na-dioctylsulphosuccinate (10- moles of Nadioctylsulphosuccinate) in a 30 ml. centrifuge tube with a glass stopper with 10.0 ml. of a solution of tetrahexylammonium picrate in methylene chloride having the concentration of 10- moles per litre, that is moles of tetrahexylammonium picrate. Then the tube was centrifuged to separate the two phases and the absorption of the aqueous phase at 355 my. was measured.

The standard curve is obtained from the following data:

DIOCTYLSULPHOSUCCINATE Absorbance of aqueous solution at 455 nm.

Concentration of standard solution, M:

By reading the absorption obtained against a standard curve obtained in the same way the concentration can be read out. In a series of tests with twelve equal samples a standard deviation of 0.6% was obtained.

In the determination of the standard curve it was found that this was a linear relationship and passed through the origin, which shows that the reaction is quantitative.

The determination of the dioctylsulphosuccinate ion in pharmaceutical tablet preparations has been found to be extremely good. As this compound decomposes to octanol and sulphosuccinic acid, which cannot be extracted as ion pairs, this method can be used as a stability control.

EXAMPLE 3 The determination of an anion having a small extraction constant was also carried out. As anion the perchlorate ion, C10 1 was selected.

At first a standard curve was prepared by means of a number of different concentrated solutions of the perchlorate ion in a phosphate buffer, pH 6.5, in the concentration interval 10 to 10- moles per litre in the same way as was done in Example 2 above but with the difference that the concentration of tetrahexylammoniumpicrate in methylene chloride was 0.1 mole per litre.

4 The standard curve is obtained from the following data:

PERCHLORATE Absorbance of aqueous solution at 355 nm.

Concentration of standard solution, M:

7.73 X 10- 1.028 5.15X10- 0.715 3.86X10- 0.542 2.06X10- 0.320

EXAMPLE 4 The same test as in Example 3 was carried out with lauryl sulphate, at first a standard curve being obtained which was similar to the one for the perchlorate ion.

The reproducibility was good, as the determination of twelve equal samples gave a relative standard deviation of 1%.

By correcting for disturbing ions the same good result may be obtained with samples containing such ions.

In tests with solutions of sodium dioctylsulphosuccinate containing perchlorate ions as disturbing anion of the sodium dioctylsulphossuccinate has been recovered after a correction for the disturbing influence of the perchlorate ions. In tests with solutions containing perchlorate ions having chloride and iodide ions, respectively, as disturbing anion 99 and 98%, respectively, have been recovered.

The present invention is useful for the determination of anions, which together with the quaternary ammonium ion have an extraction constant exceeding about 10 Examples of such anions which can be determined are the anions of aliphatic and aromatic sulphonic acids, dialkylsulphosuccinates and higher aliphatic and aromatic carboxylic acids.

Under the circumstances given in Example 3 the method ought not to be used for substances having extraction constants below 10 Substances having constants in the range of 10 to 10 give standard curves, which cut the vertical axis and are called small constants in the application. Substances having constants exceeding 10- give curves which pass through origin and may thus be called high constants.

The extraction constant of tetrahexylammonium perchlorate is 10 which gives a satisfactorily high reaction yield for quantitative estimations, while iodide, the extraction constant of which is 10 has a too low reaction yield for quantitative use.

The present method is industrially useful as mentioned above, for example, for the stability control of sodium dioctylsulphosuccinate, but can in general be used for the determination of anions in waste water, for purity control in chemical synthesis operations and for quality control of pharmaceuticals.

We claim:

1. A method for the determination of anions by means of liquid ion exchange, characterized in that a compound selected from the group consisting of quaternary aliphatic ammonium picrates having at least 30 carbon atoms in total is used as anion exchanging compound.

2. A method according to claim 1, characterized in that tetrahexylammonium picrate is used as anion exchanging compound.

3. A method according to claim 1, characterized in that tetraheptylammonium picrate is used as anion exchanging compound.

4. A method according to claim 1, characterized in that tetraoctylammonium picrate is used as anion exchanging compound.

5. A method according to claim 1, characterized in that the quaternary ammonium picrate is used in at least 50-fold excess in comparison with the anion which is to be determined.

6. A method for the determination of anions in solution by means of ion exchange, comprising adding to a solution containing an anion in unknown quantity to be analyzed, a compound selected from the group consisting of quaternary aliphatic ammonium picrates having a total of at least 30 carbon atoms, thereby to form by ion exchange the corresponding quaternary ammonium compound of said anion to be analyzed and release a corresponding amount of picrate anion, and measuring the amount of picrate anion so released, said unknown anion being such that its said quaternary ammonium compound so formed has an extraction constant exceeding about 10 7. A method according to claim 6, wherein said quaternary aliphatic ammonium picrate is tetrahexylammonium picrate.

8. A method according to claim 6, wherein said quaternary aliphatic ammonium picrate is tetraheptylammonium picrate.

9. A method according to claim 6, wherein said quaternary aliphatic ammonium picrate is tetraoctylammonium picrate.

10. A method according to claim 6, wherein said quaternary ammonium picrate is used in at least about 50 times excess in comparison with the amount of anion which is to be determined.

11. A method according to claim 6, wherein said quaternary ammonium picrate is used in at least about 100 times excess in comparison with the amount of anion which is to be determined.

12. A method according to claim 6, wherein said solution of unknown anion is aqueous, and said quaternary aliphatic ammonium picrate added thereto is dissolved in an organic solvent immiscible with said aqueous solution, so that two layers form upon mixing, and the quaternary ammonium compound of said unknown anion becomes dissolved in said organic solvent, while said released picrate ion becomes dissolved in the aqueous phase.

13. A method according to claim 6 wherein said quaternary aliphatic ammonium picrate has a maximum of 38 carbon atoms.

14-. A method according to claim 6 wherein the aliphatic groups of said quaternary aliphatic ammonium picrate are alkyl groups.

15. A method according to claim 14 wherein said alkyl groups are identical.

16. A method according to claim 14 wherein said alkyl groups have 6 to 8 carbon atoms.

17. A method according to claim 14 wherein said alkyl groups are identical and have 6 to 8 carbon atoms.

References Cited UNITED STATES PATENTS 2,801,224 7/1957 Greer 210-37X 3,215,621 11/1965 White et a1. 210-37X MORRIS O'. WOLK, Primary Examiner R. M. REESE, Assistant Examiner US. Cl. X.R. 21021, 37

jg gg UNITED STATES PATENT OFFICE CEMNQATE 0F CORRC'HOBL Patent No. 3 ,725 ,006 Dated Apr1'1 3, 1973 T Arne E161 Brandstrom, K1 as Ragnar Gustavii U1 f mvemofls) Km ster Junuqren and Kerstin y1 berg It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

1"" -1 001mm 1 1ine 50, Su1fur1'c shou1d be Su1phonic Co1umn 2, 11'ne 30 "ion shou1d be anion Co1umn 2, 11'ne 39, "0rd" shou1d be org Co1umn 3, 11"ne 45, "455 nm" shou1d be 355 nm Signed and} sealed this 18th day of December 1973.

(SEAL) Attest:

EDWARD M. FIETCIER, JR. RENE D. TEGTMEYER Attesting Officer Acting Commissioner of Patents 

